1. Field of the Invention
The present invention relates to apparatus and methods for processing a surface of a substrate using recycled radiation. In particular, the invention relates to such apparatus and methods that employ a radiation source and a +1× system that allows radiation to be recycled through multiple cycles without returning the radiation to the source at an intensity sufficient to interfere with the radiation source's operation.
2. Description of Related Art
The fabrication of integrated circuits (ICs) involves subjecting a semiconductor substrate to numerous processes, such as photoresist coating, photolithographic exposure, photoresist development, etching, polishing, and heating or “thermal processing”. In certain applications, thermal processing is performed to activate dopants implanted into functional regions (e.g., source and drain regions) of the substrate and to modify defects in the crystalline lattice of the substrate. Thermal processing includes various heating (and cooling) techniques, such as rapid thermal annealing (RTA) and laser thermal processing (LTP). With LTP, a laser or laser diode array is used to perform thermal processing, and the technique is sometimes called “laser processing” or “laser annealing”.
Various techniques and systems for laser annealing of semiconductor substrates have been proposed and used in the integrated circuit (IC) fabrication industry. Laser annealing is preferably done in a single cycle that brings the temperature of the material being annealed up to the annealing temperature and then back down to the starting (e.g., ambient) temperature within a millisecond or less. Thermal cycle times shorter than a microsecond are readily obtained using a single pulse of radiation from a pulsed laser uniformly spread over one or more circuits and a step-and-repeat stage to expose each circuit contained on a large substrate. An example system for performing laser annealing with a pulsed laser source is described in U.S. Pat. No. 6,366,308 to Hawryluk et al.
As an alternative, continuous radiation may be used. For example, thermal processing apparatuses that employ a continuous radiation source in the form of laser diodes are described in U.S. Pat. No. 6,531,681 to Markle et al. As another example, U.S. Pat. No. 6,747,245 to Taiwar et al. describes laser annealing apparatuses that employ CO2 lasers that emit continuous rather than pulsed beams. CO2 lasers have found widespread application in metal cutting and welding applications and therefore are readily available with power levels up to 5,000 Watts.
However, known continuous radiation sources are optimally suited only for certain applications and are not easily adapted to carry out other applications. When used in semiconductor annealing applications, for example, known continuous laser diode beam sources are generally unsuited to deal with circuit density and local reflectivity variations across a semiconductor substrate. Such variations tend to lead to uneven heating of source and drain regions. While the variances in heating may sometimes be reduced by directing the beams toward the substrate at the Brewsters' angle, a 30% variation in absorption across a substrate surface may remain even under fairly ideal conditions.
Thermal processing apparatus employing CO2 laser beams may also suffer from such heating variations. Because the 10.6 micrometer wavelength of CO2 laser beams is long in comparison to any the structures likely to be found on a silicon wafer ready for the dopant activation cycle, the reflectivity at Brewsters' angle typically varies from zero to less than 4% across a typical wafer used in the fabrication of logic circuits. However memory circuits often employ metal gates and in that case the reflectivity can vary from zero to as high as 30%.
To reduce heating variations, a number of techniques have been proposed. In certain cases, a thin absorptive coating could be used to reduce reflectivity variations across a wafer. However, once a coating is applied, it may be necessary to remove it after heating. In other words, coatings necessarily add complexity to already complex manufacturing processes. Furthermore, the application and removal of coatings may result in decreasing device yields. Accordingly, there is a need for technologies that reduce variations in reflectivity without applying and stripping optical coatings from the substrate.
Optical recycling systems have also been proposed as a potential solution to the above-described problem. In general, optical recycling systems collect radiation reflected from a workpiece and reimage the collected radiation back on the workpiece. However, laser beams for welding and cutting are generally focused to a single point. For welding, cutting, and like processes, uniformity over an extended area is not an issue. Instead, efficiency is an issue in such processes. Accordingly, cutting and welding recycling systems can employ a simpler −1× optical recycling system. For example, a welding recycling system may employ a −1× reflective element in the form of a spherical mirror arranged so its center of curvature coincides with the focal point of the laser beam.
In contrast, laser annealing systems for semiconductor substrates tend to employ line or other elongate images that simultaneously heat extended areas on the surface of substrates. Such extended areas may include regions of both high and low reflectance. Accordingly, it is essential that any optical recycling system used for laser annealing apparatus return the reflected radiation back to its point of origin on the substrate and not merely back to another point on the beam image. In other words, the recycling system for such laser annealing systems must collect and reimage radiation from the substrate in a +1× manner. Exemplary +1× systems are described, e.g., in U.S. Pat. No. 7,154,066 to Talwar et al., U.S. Patent Application Publication No. 20050045604 to Talwar et al., and U.S. Patent Application Publication No. 20050189329 to Talwar et al.
Nevertheless, additional opportunities exist to improve recycling systems for laser annealing applications. Such improvement involves overcoming a number of technological challenges via novel and non-obvious ways as described herein.